An inboard boat is a boat where the engine is within the hull of the boat and the propeller shaft extends from the engine to the underside of the boat. These boats can range in size from small personal “runabouts” to huge ocean liners. Most inboard boats that are of the size between runabouts and ocean going yachts have the exhaust gasses exiting the boat at the stem (or rear) of the boat. Typically, the exhaust outlets protrude from and terminate at the transom (or rear component) of the boat.
As these boats travel forward, the boat creates an eddy current in the air that tends to draw the boat engine's exhaust gasses forward, into the boat (the station wagon effect). If the boat's engine is gasoline powered, the exhaust likely contains dangerous levels of carbon monoxide. There have been deaths and illness caused from this ingress of boat exhaust into boats. Young children and the unborn fetus are particularly at risk. If the boat's engine is diesel powered, the exhaust generally has lower levels of carbon monoxide than that produced by a gasoline engine, but contains high levels of sulfur oxides, which can cause a person to feel ill.
People have operated inboard boats for decades. It is only within the past few years that significant concern about the dangers of carbon monoxide poisoning from the operation of these boats has been raised. Because of this growing awareness of the problem, many drowning victims are now tested for blood levels of carboxyhemoglobin to determine if carbon monoxide was a factor in their drowning.
There is also an activity called “platform dragging”, which has also been called “teak surfing” or “teak dragging”, wherein a person will hang onto the swim platform at the rear of an inboard boat while the boat is moving forward. They are sometimes able to let go of the swim platform and body surf in the boat's wake. This activity is extremely dangerous as the person in the water typically has their face in the boat engine's exhaust stream. The carbon monoxide level of a gasoline powered boat engine's exhaust is likely high enough to cause a person to lose consciousness within 2 minutes. Recent research also raises concern about the carbon monoxide exposure of people towed behind the boat, such as water skiers.
There is patent pending design for a boat exhaust system that routes the boat engine's exhaust to the side of the boat, with the boat's operator able to select which side the exhaust will exit (Fineline Industries “Sideswipe exhaust system”). The Sideswipe system is designed to minimize the carbon monoxide levels to which a person surfing in the wake of the boat is exposed. The Sideswipe system does not reduce the carbon monoxide levels, nor does it eliminate the potential for the station wagon effect; it merely routes the exhaust to one side of the boat or the other, away from a person wakesurfing behind, and to one side, of the boat.
Stern drive (inboard/outboard) and outboard boat engines have routed their exhaust through their propellers for decades. Boats with this exhaust configuration appear to be less likely to suffer from the station wagon effect.
Research is underway to develop catalytic converters for boat engines that will reduce the levels of carbon monoxide in the engine exhaust. To date, the technological hurdles have yet to be overcome and catalytic converters are not currently available for boat engines. If they do become available, due to inherent design constraints, it is very unlikely that they can be retrofitted onto existing boats.